I moved into an old (c. 1920s) rental apartment building in the northeastern US (Philadelphia) and would like to hang pictures and secure some furniture.
I tried putting nails in several places around the house, on internal and shared walls, but the nails only go about 0.5" deep into the plaster (I cannot find any studs). There seems to be a hard surface behind that spot. I had no such issues with external walls.
What material is most likely to be found around 0.5" into the wall?